In-situ experimental study on hydro-borehole technology application to improve the hard coal excavating techniques in coal mine

The hydro-mining technology is considered as a promising method of bituminous coal excavation. The paper presents the results of the in-situ experimental campaign and modelling of hydro-cutting technology application. The proposed innovative technology was tested in terms of the effects of the distance between the outlet of water from the nozzle and a sidewall, pressure of the water jet, as well as the type of a nozzle on hydro-mining effectiveness. The hydro-cutting tests of coal seam performed in the Experimental Mine “Barbara” in Poland proved that the increase in water pressure in the range 20–40 MPa only slightly affects the coal face structure, while high pressure, of 80–100 MPa, has a significant impact on a coal face structure. The experimental results also showed the major effects of operating time as well as the distance of the water jet on the effectiveness of coal face mining.


Materials and methods
The range of the most important parameters, i.e. water pressure and its output, which allow coal to be cut with satisfactory efficiency were determined. It was proved that the optimal parameters for effective hydro-mining of bituminous coal are the following: the pressure of up to 30.0 MPa and the flow rate of up to 500 L/min. This set of parameters was adopted in the in-situ tests as the most effective-one. In the in-situ tests the hydro-mining of hard coal was conducted in the Experimental Mine "Barbara" (Mikołów, Poland). Geological structure of the rock mass dates back to Quaternary period and productive Carboniferous. On the prevailing part of the seam area the volume of Quaternary varies from 4 to 6 m, in some parts of the roof of the Carboniferous appears directly below the layer of soil. Lithological formation of rocks is heterogeneous. Layers are formed from alternating fine and medium grained sandstones and shales among which numerous coal seams can be found. The most stable in terms of their expansion and volume are the seams 308, 310 and 318. The rock surrounding coal seams creates a rock mass of very heterogeneous geological properties caused by the micro and macro fissures and interchangeable appearance of mudstone and sandstone. The increase in the strength features of the rock is related to the depth. There is no vibration impact from plate tectonics. In Experimental Mine "Barbara" there are two levels located at the depth of 30 m and 46 m underground. The total length of the galleries there is almost 5 km. All of the galleries at the 30 m level were drilled in a coal seam. The thickness of this coal deposit is between 1.5 and 1.8 m.
To determine the physical and mechanical parameters of hard coal, six coal blocks used in the first stage of experiments were taken from the coal seam 310. According to the classification established for the Upper Carboniferous rocks of the Upper Silesian Coal Basin (USCB), the uniaxial compressive strength of studied coal was assessed as high. The average value of Young's modulus was set at 1254 MPa, which indicates relatively high elasticity. The physical and mechanical parameters of the coal tested are presented in Table 1, while in Fig. 1 the selected stress-strain characteristics are given.
The symbols of physical and mechanical parameters, which were used in Table 1  The gallery for the water jets cut experimental campaign in coal mine. In the in-situ experimental campaign the test station was located in the experimental gallery I on the level 30 m underground at the Experimental Mine "Barbara". For the purposes of the research 10 m of a side wall was uncovered by eliminating protective mesh and rock lining. It revealed a coal bed surface which was used for the hydro-mining tests using water jets. The station is presented in Fig. 2.
The station was equipped with a hydro cutting tool placed by the uncovered sidewall to ensure that the water jet was perpendicular to the coal face (see Fig. 3). The set of equipment was transported underground and then into the area of the conducted research with only hydro-mining tool introduced into the test station. The remaining elements of the set, that allowed proper operation of the hydro-mining tool, were located in the main gallery. Location and placement of the equipment is presented in Fig. 4.
Each of the elements in the equipment set was connected according to the scheme presented in Fig. 5.
The high-pressure pump draws water through the water filter by low-pressure hoses (blue color). From the high-pressure pump, water with a pressure of up to 100 MPa is fed through high-pressure hoses (red color) to the USO-1 device, in which a head with a nozzle is mounted. The USO-1 performs reciprocating and rotary movement of the head with nozzle, as required. The movements of the USO-1 are made possible by the oil supply from   www.nature.com/scientificreports/ a hydraulic pump through oil hoses (brown color). Both pumps are powered from an electrical switch through the corresponding cables (green color).

Computational fluid dynamics (CFD) simulation. The application of Computational Fluid Dynamics
(CFD) methods in the modelling of the transport of fluid along the nozzle requires the following input data 26,27 : -the geometry of the object, -the physical properties of the fluid, -value of the fluid stream at the inlet of the nozzle, -consideration of the initial conditions for the numerical solution, -consideration of the turbulence model for the fluid flow along the nozzle, -the time of occurrence.
The geometry of the nozzle is represented by the 3D model prepared in SolidWorks Computer Aided Design (CAD) software, while the fluid flow along the nozzle is modelled using SolidWorks Flow Simulation software using the Computational Fluid Dynamics (CFD) methods 26-29 . The Free Surface method was used in order to simulate the behaviour of fluid along the nozzle and in ambient. The method allows simulating the fluid behaviour where a gas, in the form of air, and a liquid, in the form of water, share the same area without any solid between them. The Free Surface method is based on a volumetric method called the Volume of Fluid (VOF) 26,27,30-32 . The VOF method assigns air and water as a volume fraction to each cell in the numerical grid that is simulating the domain of the numerical solution. The volume fraction of air and water always sums to 1, which means that the fraction of air implies the fraction of water and vice versa. The Flow Simulation software calculates the volume and mass of air and water leaving and entering the cells of the domain and retains mass, energy, and momentum. The transport equations are driven by initial and external boundary conditions, including gravity, as well as fluid behaviour, to obtain the actual movement of the free surface 27 .
Geometry. Figure 6 shows the view of the nozzle in the form of an assembled tool. The inlet and outlet of the nozzle were shown. The position of the carbide nozzle insert with a diameter of 0.021 m was shown in Fig. 6a. The carbide nozzle insert was used because the filtration was poor, abrasive solids were present in the fluid, and fluid flow was very high.
Numerical grid. In order to ensure that the results obtained from simulations are adequate, the numerical grid quality studies were done. The results of the analysis were shown in Fig. 7. The volume flow rate was measured at the nozzle outlet.
The effect of the mesh quality study was shown in Table 2.
According to the results of the quality study, the numerical grid will contain more than 144,503 computational cells, as shown in Table 2 and Fig. 8.
The level of detail in the results of numerical simulations depends on the accuracy and resolution of the numerical grid that is selected for CFD simulations 26 . A major challenge in CFD modelling is obtaining the numerical grid, which is characterized by high refinement level on the border of solid and fluid. Figure 8a shows the numerical grid of the nozzle with a refinement level of 2. The refinement level edits the finite element meshes in the region of flow with a little gradient in order to increase the accuracy of the numerical solution. The positions of the inlet to the nozzle and outlet to the computational domain (air) were presented in Fig. 8b.
A numerical grid shown in Fig. 8, was generated by 144,503 total cells, where 17,592 fluid cells are in contact with solids, which represents the computational domain of the fluid. The fluid volume is 0.000102 m 3 . The mesh grid was based on an orthogonal finite volume mesh. where: ρ-density (kg/m 3 ), υ velocity (m/s 1 ), p-pressure (Pa), µ-dynamic viscosity (Pa·s).
The k-εpsilon turbulence model was used to interpret the influence of occurring disturbances in the fluid transfer process in a space with a given geometry. The k-εpsilon turbulence model solution boils down to determining the value of turbulence viscosity μ t and the rate of dispersion related to energy dissipation ε caused by the occurrence of internal resistance to motion of the flowing fluid along the nozzle channel. The turbulence viscosity μ t model of the flowing fluid is expressed by an equation defined in SolidWorks Flow Simulation as follows 26 : The fluid transport equations for turbulence kinetic energy k and dispersion ε in SolidWorks Flow Simulation are expressed by the relations in the form 26 : -for turbulent kinetic energy: -for dissipation energy:  www.nature.com/scientificreports/ where: C ε1 -empirical constant, C ε1 = 1.44, C ε2 -empirical constant, C ε2 = 1.92, C µ -empirical constant, C µ = 0.09, f µ -Lam and Bremhost's damping functions, k-kinetic energy of velocity fluctuations (m 2 /s 2 ), P-eddy fluctuations, t-time (s), ε-rate of dispersion of the turbulent kinetic energy (m 2 /s 3 ), μ t -turbulent viscosity (Pa·s), σ k -the turbulent Prandtl number σ k = 1.0, σ ε -the turbulent Prandtl number σ ε = 1.3.
The nozzle is supplied with a water volume flow rate of 86 L/min (0.00143 m 3 /s) as shown in Fig. 6b. Due to the fact that the temperature between the inlet and outlet of the nozzle was different, the water parameters such as: density, dynamic viscosity, specific heat, and thermal conductivity were parameterized as functions of the temperature. The variation of physical parameters of the water at the inlet of the nozzle such as density (Fig. 9a), dynamic viscosity (Fig. 9b), specific heat (Fig. 9c) and thermal conductivity coefficient (Fig. 9d), are characterized by the corresponding graphs in Fig. 9 as a function of temperature changes, T. Figure 9 shows that as the temperature increases, the density (Fig. 9a) and dynamic viscosity (Fig. 9b) of water decrease. However, the specific heat (Fig. 9c) of water increases as the temperature increases. In case of the thermal conductivity of the water, the coefficient increases until it reaches the value of 443.50 K, and thereafter decreases to 518.16 K. The variation of the parameters of the air (fluid domain), as the environment of the nozzle, is shown in Fig. 10. Figure 10 shows that as the temperature increases, the dynamic viscosity (Fig. 10a), specific heat (Fig. 10b) and thermal conductivity (Fig. 10c) of air decrease.
The following initial boundary condition was used in numerical calculations:   Results shown in Fig. 12 allow formulating the conclusion that, at a distance of 0.05 m from the outlet of the nozzle, the velocity decreases by approximately 40% in relation to the measurement at the outlet of the nozzle. In case of the measurement of the velocity at a distance of 0.10 m from the outlet of the nozzle, the velocity decreases  www.nature.com/scientificreports/ by approximately 41% in relation to the velocity measurement at the outlet of the nozzle and by approximately 2% in relation to the velocity measurement at a distance of 0.05 m. Figure 13 shows the map of the water pressure variations at a distance of up to 0.10 m at the time interval of 180 s. It may be seen that the pressure of the water stream changes from approx. 58.5 MPa at the outlet of the nozzle to approx. 22.5 MPa at a distance of 0.10 m. Figure 14 shows The results shown in Fig. 14 allow concluding that at a distance of 0.05 m from the outlet of the nozzle, the pressure decreases by approximately 63% in relation to the measurement at the outlet of the nozzle. In case of the simulation of the pressure at a distance of 0.10 m from the outlet of the nozzle, the pressure decreases by approximately 65% in relation to the pressure simulation at the outlet of the nozzle and by approximately 4% in relation to the pressure measurement at a distance of 0.05 m. Figure 15 shows Experiments on the influence of the water jet distance from the coal face and operating time on the hydro-mining effectiveness. The effectiveness of hydro-mining depending on the distance between the outlet of water from the nozzle and a sidewall, pressure of the water jet, type of the nozzle (outlet diameter) and operation time was tested on bituminous coal from the coal seam 310 of the Experimental Mine "Barbara". The research was conducted in 3 configurations (that is distances from the sidewall: 0.00, 0.05 and 0.10 m). In each of them the pressure used was 100 MPa and water flow was constant-86 L/min. Two types of nozzle output diameters were used for the tests: 2.1 mm and 2.5 mm. Each test took 180 s and every 60 s a measurement of the depth in the mined coal face was done. In Fig. 16 the outline of the nozzles placement during the www.nature.com/scientificreports/ hydro-mining tests was presented. The results of the studied tests of the hydro-mining effectiveness in terms of water jet distance from the coal face as well as the operating time were presented in Table 3. The depth was measured with caliper between bottom of the slot and surface of the coal. The results obtained allow concluding that the effectiveness in hydro-mining of the coal bed was low. Water test was recognized to be rather cutting the coal face than mining it. A significant influence of the distance between the nozzle output and the coal bed could be observed. A dependency between the angle of the water jet towards coal stratification-cleavage surface was also observed. The water jet impact in parallel to the coal stratification causes its fragmentation, crushing, and hence significantly increases the effectiveness of the mining at the coal face. In Fig. 17 the influence of high pressure water jet on impact in parallel to the coal stratification and distance from the coal face was presented.
Research on the influence of the water jet distance from the coal face and operating time on the hydro-mining effectiveness. The aim of this part of the experiments was to determine the minimum value of the water jet pressure that would allow coal mining. The tests were carried out with a nozzle diameter of 2.5 mm. The water jet was in a perpendicular position towards the coal face. The research was conducted for the following pressures: 20, 40, 60, 80 and 100 MPa, respectively. In each test the distance between the water jet and the coal face was fixed at 0.05 m. The measurements were taken after in a set time intervals of the experiment duration. The results are presented in Table 4.
The results presented in Table 4 indicate that the pressure values between 20 and 40 MPa slightly impact the coal face structure. Visible increase in the effectiveness of water test is observed for higher pressure values, of 80-100 MPa. Moreover, it can be noticed, that the effectiveness of water test impact is the highest within the first 180 s of its operating. After this time a lack of mining (cutting) progress can be seen. www.nature.com/scientificreports/

Conclusions
• The hydro-cutting and hydro-mining are complex processes because the dynamic impact of a stream of water injected under high pressure and hitting a hard surface of coal. • In the paper the new, innovative hydro-mining technique was proposed. It could be successfully applied in the hard coal mines but the proposed solution of hydro-mining needs to be calibrated to the specific conditions of each hard coal mine. • The experimental tests on the effects of various pressures of the water jet on the effectiveness of coal face mining reveal, that the increase in pressure between 20 and 40 MPa only slightly impacts the coal face structure. The significant increase in the effectiveness of water tests was observed at higher pressure values, of 80-100 MPa. • The experimental tests on the influence of the water jet distance from the coal face and operating time on the hydro-mining effectiveness reveal, that distance of the water jet significantly affects the effectiveness of coal face mining. Namely, the water jet impact in parallel to the coal stratification causes its fragmentation, crushing, and hence significantly increases the effectiveness of the mining at the coal face. The distances of 0.05 and 0.10 m from the outlet of the nozzle decrease the cutting force by approximately 63% and 65%, respectively, in comparison with the measurement at the outlet of the nozzle.     www.nature.com/scientificreports/

Data availability
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